ARM: dts: imx27: Add core voltages

This patch adds core voltages for i.MX27 CPUs. Only 266 and 400 MHz modes
is documented in the datasheet, so we add a 266 MHz frequency for conform this.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>